Thermodilution techniques can be used to measure cardiac output. In thermodilution, a bolus of cold saline injectate is injected into the blood steam and allowed to mix with the blood. The resulting temperature change of the mixture can then be used to calculate cardiac output.
It is known to inject the injectate into the vena cava or the right atrium and, in some cases, the right ventricle and to measure the resulting temperature change with a temperature sensor, such as a thermistor, in the pulmonary artery. These functions are commonly carried out with a thermodilution catheter.
For best results for computing cardiac output, it is only necessary that the injectate mix thoroughly and completely with the flowing blood prior to contacting the thermal sensor. Without proper mixing, the cardiac output computed from the thermal curve will not be reliable.
Theoretically, it is possible to use a fast response catheter and thermodilution techniques to calculate right heart ejection fraction. For the best results in calculating right heart ejection fraction, it is necessary that the injectate enter the right ventricle in an abrupt manner as opposed to a slowly decreasing rate. It is also necessary that this injectate be evenly distributed and well mixed throughout the right ventricular volume prior to ejection into the pulmonary artery. When this is done, the temperature sensed in the pulmonary artery represents the mean right ventricular temperature. The prior art has unsuccessfully sought to meet these conditions by injecting directly into the right ventricle and measuring the resulting thermal curve with a thermal sensor which has a relatively rapid response. The lack of success with this approach lies primarily with the mixing requirement described above. Examination of the thermal curves for this prior art approach indicates that the injectate is not evenly distributed and well mixed throughout the ventricle. In addition, these fast response catheters function poorly for determining cardiac output as a result of streaming (uneven distribution and poor mixing) of the injectate. Also, the probability of inducing arrhythmias, especially premature ventricular contractions, is much higher.
The temperature of the mixture in the pulmonary artery changes with each discharge from the right ventricle in a generally step-wise fashion. In order to use thermodilution techniques successfully for the measurement of right heart ejection fraction, it is necessary to measure the beat-to-beat temperature differences of the mixture. Accordingly, it is desirable to use a fast response thermistor, and one such thermistor is shown in a co-pending application Ser. No. 570,619, of William Webler entitled Apparatus With Fast Response Thermistor and filed on even date herewith. However, the use of a fast response thermistor alone is insufficient due to the tendency of the thermistor to contact the wall of the pulmonary artery. This insulates and shields the thermistor from the flowing mixture and prevents the thermistor from accurately and rapidly measuring the temperature of the mixture.